This invention relates to semiconductor devices and, more particularly, to solid state devices such as laser diodes that require the use of heat sinks for proper operation.
The manufacture of semiconductor devices typically involves the growth of epitaxial layers on a suitable substrate either by vapor or liquid phase metalo-organic chemical deposition, followed by various steps that may employ different combinations of photolithographic resist masking, the etching away of certain layers and the overgrowth of other layers of semiconductor material, insulating oxides and metalization.
In laser diode semiconductor devices, high current densities are employed that generate heat. The internally generated heat must be transferred through a semiconductor surface interface to an external heat sink. At the interface, the solder that bonds the chip to the heat sink provides both an electrical and heat transfer path. Typical solder materials include indium, gold-tin, silver-tin, and lead-tin. Heatsinks are made of such materials as copper or copper-tungsten or metallized, thermally conductive ceramics such as alumina, berylia, diamond, and cubic boron nitride. In one commonly used process, the solder metal e.g., indium, is evaporated onto the heatsink surface in chamber using a standard e-gun or thermal evaporator. Then the solder is removed from the evaporator. The chip is placed on top of the solder coated heatsink, and a weight or spring force is used to hold the chip in place. The assembly is run through a temperature and atmosphere controlled cycle to xe2x80x9creflowxe2x80x9d the solder and then cooled to solidify the solder so that the chip is affixed to the heatsink.
An alternative soldering procedures involves the use of solder xe2x80x9cpreformsxe2x80x9d. When solder preforms are used, the heat sink is placed in an oven and a preform is placed on the heat sink with the chip on top of the preform. The oven is heated until the preform melts and then the assembly is cooled to affix the chip to the heat sink.
It is well known that surface oxidation and/or contamination can lead to poor wetting of the solder at the surface interface between the heat sink and the chip, or xe2x80x9cdiexe2x80x9d. Despite the use of efficient heat sink structures, careful cleaning of the die, the use of appropriate solders and the employment of drive currents in operation that are within prescribed ranges, some device failures continue to occur. The several watts of heat produced by a typical high-power semiconductor laser device emanates from an area that may typically be of the order of 0.1 mmxc3x972 mm, producing on the order of 1-5 kilo-watts cm2 of heat flux.
Microscopic analysis of devices that have failed due to overheating sometimes reveals the existence of gaps in the solder bond between the semiconductor device and the heat sink. Such gaps reduce the amount of interface area available for effective heat transfer and permit critical overheating in localized areas. Even a small void in the solder-bond can give rise to hot spots inside the laser device under these conditions. Local hot spots encourage the growth of defects that shorten the life of the device. They also produce local refractive index variations that can degrade the output beam quality in the case of a laser device.
We have discovered that localized overheating between the semiconductor device and the heat sink tends to occur when gasses are entrapped during the process of soldering the chip to the heat sink. Entrapped gasses prevent the even reflow of solder. In accordance with the principles of our invention, one or more gas vents are provided in the interface surface geometry to prevent the formation of solder voids during the soldering process. Advantageously, the provision of such vents in the interface surface geometry may be effected by the use of appropriate masking and etching of the semiconductor device following the epitaxial re-growth process. Alternatively, gas vents may be provided in the heat sink or in the solder preform. The use of such xe2x80x9cdiebond ventsxe2x80x9d also allows the chamber in which soldering is performed to be pressurized so as to force molten solder into the interface without danger of entrapping gas that would cause a solder void in the interface.